In most current wireless communication systems, nodes in a network may be configured to operate based on a single transmit and a single receive antenna. However, for many of these current wireless systems, the use of multiple transmit and/or receive antennas may result in an improved overall system performance. These multi-antenna configurations may be utilized to reduce the negative effects that multi-path and/or signal interference may have on signal reception. Existing systems and/or systems which are being currently deployed, for example, CDMA-based systems, TDMA-based systems, WLAN systems, and OFDM-based systems such as IEEE 802.11a/g/n, may benefit from configurations based on multiple transmit and/or receive antennas. It is anticipated that multiple antenna techniques may be increasingly utilized in connection with the deployment of base station infrastructure and mobile subscriber units in cellular systems to address the increasing capacity demands being placed on those systems. These demands arise, in part, from a shift underway from current voice-based services to next-generation wireless multimedia services that provide voice, video, and data communication.
The utilization of multiple transmit and/or receive antennas is designed to introduce a diversity gain and array gain and to suppress interference generated within the signal reception process. Such diversity gains improve system performance by increasing received signal-to-noise ratio, by providing more robustness against signal interference, and/or by permitting greater frequency reuse for higher capacity. In communication systems that incorporate multi-antenna receivers, a set of M receive antennas may be utilized to null the effect of (M−1) interferers. Accordingly, N signals may be simultaneously transmitted in the same bandwidth using N transmit antennas, with the transmitted signal then being separated into N respective signals by way of a set of N antennas deployed at the receiver. Systems that utilize multiple transmit and multiple receive antenna may be referred to as multiple-input multiple-output (MIMO) systems. One attractive aspect of multi-antenna systems, in particular MIMO systems, is the significant increase in system capacity that may be achieved by utilizing these transmission configurations. For a fixed overall transmitted power, the capacity offered by a MIMO configuration may scale with the increased signal-to-noise ratio (SNR).
However, the widespread deployment of multi-antenna systems in wireless communications, particularly in wireless handset devices, has been limited by, among other things, increased power consumption, increased size, increased complexity, and increased cost. These limitations are a direct result of the need to provide a separate RF chain for each transmit and receive antenna of multi-antenna systems. Each RF receive chain generally comprises a tuner/filter, down-converter, and an analog to digital converter. Each RF transmit chain generally comprises at least one oscillator, modulator, and amplifier. It is therefore apparent that as the number of transmit and receive antennas increases, the size, complexity, power consumption, and overall cost may increase.
As stated above, an RF transmitter may comprise at least one modulator. This modulator may perform a modulation scheme which requires impressing both phase and amplitude information onto a carrier. These modulation schemes have traditionally been performed using quadrature modulation whereby an in-phase carrier (I) and a quadrature carrier (Q) are amplitude modulated and then combined prior to transmission. In this manner, when using quadrature modulation, the amplitude modulation and phase modulation occur simultaneously. From the preceding discussion, it may be seen that quadrature modulation make use of Cartesian coordinates x and y, wherein the x axes is the I (in-phase) axis and the y axis is the Q (quadrature) axis.
In operation, an RF transmitter and/or receiver may require signals at a multitude of frequencies. Traditionally these signals are generated through the use of voltage controlled oscillators (VCO) and phase locked loops (PLL). One drawback of using VCO and PLL for frequency generation is that these circuits have a relatively narrow range of operating frequencies. The narrow range of operation often results in the need for many VCO and/or PLL in a single receiver or transmitter.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.